1. Field of the Invention
The present invention relates to an irradiation pattern data generation method and plotting system used in irradiating energy upon an object of plotting to carry out plotting, and to a method of fabricating a photomask formed by the plotting system.
2. Description of the Related Art
Techniques are known for using a plotting device to irradiate energy such as an electron beam to form a desired pattern on an object of plotting. Such techniques are applied in, for example, the manufacture of photomasks used in the photolithographic steps in the fabrication process of semiconductors.
A raster scan method and a variable rectangle method are known as methods of such a plotting device. The raster scan method is a plotting method of successively scanning each of the coordinates on a mask by a point beam that has been concentrated to a point. In this raster scan method, a desired pattern is formed on a by turning the beam ON at coordinates for which there are data and turning the beam OFF at coordinates for which there are no data. The advantage of the raster scan method is the ability to estimate the plotting time substantially by the size of the plotting region. All of the coordinates of the plotting region are scanned regardless of the presence or absence of data, and the plotting time therefore depends on the size of the plotting region. However, the difficulty of implementing optical proximity effect correction in the raster scan method necessitates the use of an electron beam of relatively low acceleration voltage (on the order of, for example, 20 keV) that does not exhibit the influence of the proximity effect. Obtaining sufficient resolving power in the raster scan mode is problematic due to the use of an electron beam of relatively low acceleration voltage.
In contrast, the variable rectangle mode is a plotting mode in which the irradiation pattern data are represented by rectangles and these rectangles then exposed one at a time. In a plotting device of the variable rectangle mode, for example, two L-shaped apertures are combined to form a rectangular electron beam of any size. In this method, the acceleration voltage of the electron beam can be made relatively high (on the order of, for example, 50 keV) and a high resolving power can therefore be obtained on the object of plotting. In addition, although the proximity effect cannot be ignored when an electron beam of high acceleration voltage is used, the dimensional accuracy of a micropattern can be increased by correcting the proximity effect by using a function of the area density of a pattern to vary the amount of exposure during plotting.
Of the above-described plotting device modes, the variable rectangle mode, which is advantageous in terms of dimensional accuracy, has currently become the mainstream. When using the variable rectangle method, however, the plotting time can become time-consuming in some cases. This lengthy plotting time occurs because the plotting time increases, not in proportion to the original data (hereinbelow referred to as the “design pattern”), but rather, substantially in proportion to the number of rectangles in the data (hereinbelow referred to as the “irradiation pattern”) in which patterns have been divided for plotting. In particular, the inclusion of a pattern that extends diagonally in the design pattern results in division into a multiplicity of extremely minute rectangles in the irradiation pattern, whereby the plotting time becomes extremely long.
A technique that enables a reduction of the plotting time is therefore desired in the variable rectangle method.
The dimensional accuracy during plotting may fall when the form of the rectangles becomes extremely minute. A technique that enables an increase in dimensional accuracy during plotting is therefore also desired.
In relation to the foregoing explanation, JP-A-2000-241958 discloses a photomask provided with a transparent substrate and a shield pattern that indicates a polygonal circuit pattern that includes diagonal lines as polygons in which the diagonal lines are represented as stepped shapes by a plurality of rectangles and in which the width R of the rectangles is in the range Rw<R<(Rp×m) (where m is the transfer magnification of the exposure device, Rp is the resolving power of the exposure device, and Rw is the resolving power of the mask plotting device).
However, a dimensional shift (hereinbelow referred to as “process bias”) may occur between the pattern of the electron beam that is irradiated upon the object of plotting and the pattern that is actually formed on the object of plotting. This dimensional shift depends on such factors as the electron beam resist material that is formed on the object of plotting, the etching equipment, the fabrication process of the material and film thickness of the object of plotting, the dimensions of the pattern, and the spacing between adjacent patterns. Thus, to form a pattern of desired dimensions on an object of plotting, the electron beam must be irradiated by a correction pattern in which correction has been carried out based on the amount of change, which is the process bias. This type of correction is carried out by, for example, a rule-based method (a method in which pattern dimensions and a table or a function of the amount of correction from adjacent patterns are first generated and correction then carried out by successively applying the pattern to this table or function).
In relation to the foregoing explanation, the publication JP-A-2003-273001 discloses a data generation method for plotting a mask pattern by means of an electron beam. In this data generation method, a bias process for dimensional correction is applied to pattern data of a semiconductor integrated circuit that has been subjected to design layout, wherein correction is applied to pattern data that have been divided into rectangles and/or trapezoids without alteration and without subjecting pattern data that have been divided into rectangles and/or trapezoids to an outline process.
When carrying out the above-described correction of process bias in portions corresponding to diagonal patterns of a design pattern in the above-described plotting device of the variable rectangle method, the number of rectangles may undergo a further dramatic increase. The reasons for this increase are next explained with reference to FIGS. 1A-1C.
FIG. 1A shows a rectangular approximation pattern in which the diagonal pattern portion of a design pattern is approximated by rectangular shapes for the variable rectangle method. FIG. 1A shows a rectangular approximation pattern made up from four rectangles. Each rectangle is formed from parallel sides in the X-axis direction and parallel sides in the Y-axis direction. The side portions of the design pattern are represented by a stepped shape by this plurality of rectangles.
FIG. 1B is a view for explaining the implementation of the process bias correction of the rectangular approximation pattern. In FIG. 1B, a “+(plus)” value process bias is conferred to the rectangular approximation pattern, and the dimensions of the rectangular approximation pattern shows a state that is enlarged to the extent of this process bias from the dimensions of the design pattern. In this pattern that results from correction, the Y-coordinates of the apices that form the stepped shapes on the two side portions of the design pattern do not coincide. As a result, the pattern that follows correction cannot be represented by just four rectangles as in the rectangular approximation pattern. Accordingly, to represent the pattern following correction, a greater number of rectangles are necessary. In other words, the implementation of the process bias correction causes the number of rectangles that form the irradiation pattern to surpass the number of rectangles that form the rectangular approximation pattern. The increase in the number of rectangles leads to an increase in the plotting time.
FIG. 1C shows the representation of the pattern following correction by rectangles. When actually plotting, electron beam irradiation is carried out for each of the rectangles r1, r2, and r3 shown in FIG. 1C. Here, when the process bias Δ is a “+” value, i.e., when the dimensions of the rectangular approximation pattern are made greater by the process bias Δ than the dimensions of the design pattern, long and narrow rectangles r2 that form regions that include the sides in the X-axis direction of the rectangular approximation pattern have a width in the Y-axis direction that is just twice the length of the process bias. In other words, rectangles r2 are minute rectangles. Irradiating an electron beam in such minute rectangles r2 is disadvantageous from the standpoint of dimensional accuracy.
It is therefore an object of the present invention to provide a irradiation pattern data generation method, a mask fabrication method, and a plotting system that enable a reduction of the plotting time.
It is another object of the present invention to provide an irradiation pattern data generation method, a mask fabrication method, and a plotting system that do not cause an increase in the number of rectangles when implementing process bias correction.
It is yet another object of the present invention to provide an irradiation pattern data generation method, a mask fabrication method, and a plotting system that limit the miniaturization of rectangles.